In composite transmissions of the above-described type, it is a common practice to provide a hydrostatic branch which comprises two hydrostatic machines or units, usually of the axial-piston type, which can be connected in a closed hydraulic circuit to form a hydraulic transmission which may provide the variable speeds or transmission ratios over each of the selected ranges of the transmission. Selection between ranges may be effected in the mechanical branch of the transmission which usually comprises a multistep or plural step summing transmission of the planetary gear type. Clutches or brakes can select the speed ranges of the mechanical transmission and, within each speed or ratio range the hydrostatic transmission can be adjustable in a stepless manner to that the overall transmission system, i.e. the composite transmission is steplessly variable from a minimum transmission ratio to a maximum.
In such systems, moreover, the clutches for speed selection may connect the second unit of the hydrostatic transmission to one or more members of the planetary-gear summing transmission and the second hydrostatic unit can be of a constant displacement type. In this case, of course, the first hydrostatic unit is of the variable displacement type. The term "displacement" is used here to refer to the fluid displacement per revolution and, naturally, the displacement represents the transmission ratio of the hydrostatic transmission or the hydrostatic branch of the composite transmission. The variable displacement unit can be shifted from a zero displacement state in opposite directions to opposite limits, referred to as the positive machine displacement and negative machine displacement, respectively, representing opposite senses of rotation of the shaft of the second or constant-displacement unit. The term "negative" is here used in the sense of a reverse rotation of the shaft or drive of the system in the reverse sense (as contrasted with forward or "positive" drive).
A composite transmission having a hydrostatic branch and a multi-shaft planetary transmission forming a summing transmission, is described, for example, in German Pat. 2,415,002 corresponding in part to U.S. Pat. No. 3,913,325.
The mechanical transmission branch is provided with a reversing transmission at its input side so that in reversing of the setting of this transmission, all of the parts of the mechanical branch connected with the input shaft must be driven even for reverse operation of the load or vehicle in which the transmission is provided. This has been found to be a significant disadvantage of this transmission since all of the rotary masses must be entrained in senses opposite their normal rotation, thereby contributing significantly to the wear of the system and to the inertia thereof.
In addition, such an arrangement requires a simultaneous compensation of the throughputs of the two hydrostatic units so that the variable-displacement hydrostatic units must be shifted from one limiting setting to the opposite limiting setting whenever the directions change is effected.
The system also operates with a jaw-type clutch which has also been found to be disadvantageous since its operation must be synchronized with the opening of both of the range-selection clutches to minimize wear of the noise and stress upon the transmission. Such transmissions have been found not to be fully effective for reversible equipment (vehicles).